Color digital halftoning using black and secondary color replacement

ABSTRACT

This system selectively enables a printer to deposit primary or black color dots at pixel locations on a sheet to produce a color image--in such a way as to increase the overall number of dots used in the image, thereby minimizing contrast between dots on the sheet and adjacent areas and so providing a better simulation of a continuous-tone image. The result is print quality often near that of error diffusion, usually better than any dither technique, but with the greater speed of a dither technique. The system includes a memory for storing a first color vector for each pixel in the image, respectively, and a processor coupled to the memory for determining a second color vector from each of the first vectors, respectively. The processor accomplishes this determination by replacing a fraction of the black color component of the first vector with primary colors. Preferably the processor also removes from the first color vector a quantity of white, in conjunction with the black replacement; in this case the processor sets the fraction of the black component to a value between zero and one inclusive, depending on the quantity of white that is available, in the first color vector, for removal. Preferably the fraction is the smaller of (1) the quantity of black present and (2) a certain fraction of the quantity of white available for removal. Analogously a fraction of the secondary color component of a color vector is replaced with primary colors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of Ser. No. 08/438,294, filed on May 10, 1995 andnow U.S. Pat. No. 5,657,137, which was a file-wrapper continuingapplication of Ser. No. 08/187,935, filed on Jan. 27, 1994 (and nowabandoned); which in turn was a continuation-in-part of Ser. No.07/878,931 filed May 4, 1992 and now issued on U.S. Pat. No. 5,377,024,a continuation-in-part of copending commonly assigned U.S. patentapplication APPARATUS FOR FORMING COLOR IMAGES USING A HUE-PLUS-GRAYCOLOR MODEL by PAUL H. DILLINGER, Ser. No. 07/878,931, filed May 4,1992, and now issued as U.S. Pat. 5,377,024; and is related to thefollowing copending and commonly assigned U.S. patent applicationsHALFTONE IMAGES USING PRINTED SYMBOLS MODELLING, by Qian Lin, Ser. No.08/057,244, filed May 3, 1993; HALFTONE IMAGES USING SPECIAL FILTERS, byQian Lin, Ser. No. 08/060,285, filed May 11, 1993; BI-LEVEL DIGITALCOLOR PRINTER SYSTEM EXHIBITING IMPROVED UNDERCOLOR REMOVAL AND ERRORDIFFUSION PROCEDURES by Gary Dispoto, et al., Ser. No. 08/187,567, filedJan. 27, 1994; ADAPTIVE COLOR RENDERING BY AN INKJET PRINTER BASED ONOBJECT TYPE by Thomas G. Smith, et al., Ser. No. 08/189,006, filed Jan.27, 1994; COLOR HALFTONING OPTIONS INFLUENCED BY PRINT MODE SETTING, byKirt A. Winter, et al., Ser. No. 08/187,933, filed Jan. 27, 1994;MANUAL/AUTOMATIC USER OPTION FOR COLOR PRINTING OF DIFFERENT TYPES OFOBJECTS, by Sachin S. Naik, et al., Ser. No. 08/187,942, filed Jan. 27,1994; and AUTOMATED OPTIMIZATION OF HARDCOPY OUTPUT by Steven O. Miller,et al., Ser. No. 08/188,618, filed Jan. 27, 1994; which are hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates to digital color printers which produce binarylevel color dots to create a full-color representation of an image and,more particularly, to a digital color printer performing color digitalhalftoning on a color vector using noise dithering and replacement ofblack and secondary colors with primary colors.

BACKGROUND OF THE INVENTION

Currently, both inkjet printers and laser printers are capable ofproducing full color images with high quality and precision. Such colorprinters are controlled by a printer driver program which provides aninterface between an application program running on a host processor andthe printer. Normally, user creates a document using an application onthe host computer and then calls for initiation of the printer driverprogram. In response to the user's instituting a print command, the hostcomputer transmits a series of page descriptions to the printer driver.The printer driver then proceeds to use built-in functions to rasterizethe page description into a pixel map of a predefined resolution (e.g.300 dots per inch, 600 dots per inch, etc.). The printer driver mustalso adjust the printed colors to match the screen colors as closely aspossible.

Personal computers (PC's) use eight-bit values to designate each primarycolor. To produce secondary colors, a PC uses combinations of the threeeight-bit values to control the computer's display device (e.g. a colorCRT). A 24 bit value can represent 2²⁴ different color values, whichcolor values can be reproduced by appropriate control of the CRT's colorelectron guns. When a color printer is called upon to accuratelyreproduce that many color values, extensive color processing isrequired.

Thus, in order to reproduce a received color value, a color printer mustconvert the color value into a color command that is recognized by theprinter engine. It was early realized that it was impractical to producea color table map that mapped all 2²⁴ possible PC-generated input colorsto printer engine color codes.

Each pixel in the pixel map comprises, for instance, three eight-bitvalues corresponding to red, green and blue values derived from or for adisplay device in the host processor. The printer driver must adjust thecolor values in accordance with a predetermined calibration function soas to assure that the to-be-printed colors will appear the same as, orbear some preselected relation to, the colors displayed on the displaydevice.

This is performed by a color management or mapping system that assuresthe colors produced by one product (a printer, scanner, monitor, filmrecorder, etc.) match or relate as desired to those produced on others.Color management systems typically have two components, "profiles" ofindividual color products that specify the color capabilities of thedevice, and software that runs on a host computer that uses thisinformation to ensure that the colors produced by one product matchthose produced by another. In cases where a particular color is notwithin the color gamut of a target device (i.e. the target device simplyis incapable of reproducing the color), the color management softwaremust provide a close match. Device independent color is a termdescribing a computer system capable of reproducing a color accuratelyon any attached color device (printer, monitor, scanner, etc.).Device-independent color is usually implemented by developing "deviceprofiles" that describe the colors a product can produce and bydeveloping a color matching engine that uses the profiles to convertcolor data to assure a match between devices.

RGB is a color space that uses as its primary colors red, green, andblue. These three colors are the primary "additive" colors. In devicesthat use projected light to produce an image (for example, televisionsor computer monitors), a spectrum of colors can be reproduced using red,green, and blue. Red and green combine to form yellow, red and blue toform magenta, green and blue to form cyan, and all three to form white.Any other shade can be produced by combining different amounts of thethree primary colors.

CMYK is a color space that uses as its primary colors cyan, magenta,yellow and black. These four colors are the primary "subtractive"colors, that is, when printed on paper, the CMYK colors subtract somecolors while reflecting others. Cyan and magenta combine to form blue,cyan and yellow to form green, magenta and yellow to form red, and intheory, all three to form black. However, it is sometimes difficult toget a satisfying black using a given set of cyan, magenta, and yellowpigments, so many reflective color-based products add a "true" blackcolor, hence CMYK, not CMY. (To avoid confusion with blue, the letter Kis used to represent black) . The CMYK color set is sometimes called"process color."

In printing the printer uses the three subtractive primary colors. Theyare called subtractive because in each, one of the three additive colorshas been subtracted from the white light. When red is subtracted, greenand blue are left which combine to form the color cyan. When green issubtracted, red and blue are left which combine to form the colormagenta. When blue is subtracted, red and green light combine to formthe color yellow. The printer's subtractive primary colors are cyan,magenta, and yellow. The overprinting of all three in solid imagesyields black. The combination is black because each has subtracted oneof the three additive primary components of white light and the completeabsence of light is black.

Thus, the printer driver must convert the red, green and blue values toCyan (C), Magenta (M), and Yellow (Y) values. As a result, each pixel isthen represented by three eight-bit values which identify thecorresponding levels of C,M,Y that will be used to subsequently controlthe print mechanism. An additional eight-bit value is supplied for apixel black (K) dot to be applied at the pixel location.

Color printers can print one of eight colors at a particular pixel (red,green, blue, cyan, magenta, yellow, black, or white). However, thecomputer can request any one of 16 million colors. Therefore, it isnecessary to produce a translation between 24-bit pixels (16 millioncolors) and 3-bit pixels (eight colors). This translation is calleddigtal halftoning. It is an integral part of color printing.

Digital halftoning refers to any process that creates the illusion ofcontinuous tone images by judicious arrangement of binary pictureelements, such as ink drops in the case of inkjet printers. Thus,halftoning is printing the simulation of a continuous-tone image, suchas a shaded drawing or a photograph, with groups or cells of color orblack dots. The dots are placed in such a way that they appear to thehuman eye to be a single color. Digital halftoning is sometimes calledspatial dithering.

Printing presses and most printers use halftoning to render images. Onprinting presses, different size dots can be used to produce differentshades of gray or color. Most color printers are binary in nature, inthat they either apply a full color dot or no color dot to a pixellocation. Such color printers do not employ a control mechanism toenable adjustment of the intensity of a particularly applied color dot.In binary printers, different patterns of identical dots are used toproduce halftone images. As a result, a printer driver for a binarycolor printer employs a color digital halftoning process which reducesthe 24 bit color information to 3 bits per pixel print position (1 bitfor each of the C,Y, and M color planes).

Dithering can be used to reproduce gray shades using only black ink, orthe full spectrum of color using only the process colors (cyan, magenta,yellow, black). For example, to produce green, a color printer lays downpatterns of small yellow and cyan dots that appear to the eye to begreen. There are many halftoning techniques, each with its own methodfor laying down dots. Examples include pattern dithering and errordiffusion.

Pattern dithering uses a library of set patterns to reproduce a color(in color printing) or a gray shade (in monochrome printing). Patterndithering can be characterized as ordered or random. Ordered dithersgenerally fall into one of two broad classes, dispersed and clustered.

In dithering, a lot of work has been done to create the ideal "dithercell". This effort has been put into developing dither cells that haverandom or "blue noise" characteristics. Such "super-smooth" dither cellsproduce an image that appears almost as good as error diffused, but withthe speed performance of a dither. See U.S. patent applications HALFTONEIMAGES USING PRINTED SYMBOLS MODELLING, by Qian Lin, Ser. No.08/057,244, filed May 3, 1993; and HALFTONE IMAGES USING SPECIALFILTERS, by Qian Lin, Ser. No. 08/060,285, filed May 11, 1993.

Dithers are implemented by use of a dither cell or dither matrix orthreshold array, also called a mask, a two dimensional matrix ofthresholds. Pixel values are compared to corresponding entries in thedither cell to determine if they should be turned on or off. In this waya shade of red for example can be converted to full red or no red. Manydifferent approaches exist that vary the size of the cell and thedistribution of the thresholds. Thus, halftoning is accomplished by asimple pointwise comparison of the input image against a predeterminedthreshold array or mask. For every point or pixel in the input image,depending on which point value is larger, the image or the mask, eithera 1 or 0, respectively, is placed at the corresponding location in thebinary output image.

Pattern dithering in general benefits from ease of implementation.Pattern dithering is computationally fast but does not offer the bestpossible reproduction quality. Error diffusion is a technique for layingdown dots of the three process colors to produce the full spectrum ofcolor. Error diffusion techniques use complex algorithms to lay downdots of color in a random rather than a repeated pattern, which improvesthe quality of the image. Error diffusion makes the best approximationit can for a given pixel, calculates how far that approximation is fromthe ideal and propagates this "error" to neighboring pixels. In this waya given pixel may not be particularly accurate, but the area is. Ingeneral, error diffusion generates much better print quality thandithering. However, typically, intense calculation is required to createthe random pattern, so printing images using error diffusion is muchslower than using pattern dithering.

Thus, halftoning algorithms can generally be evaluated in terms of speedof execution and resulting print quality. Often a tradeoff needs to bemade between an algorithm that is fast but does not produce optimumprint quality versus an alternative approach with better print qualitythat takes longer. So the problem each halftoning technique is trying toaddress is how to quickly produce a high print quality image. There is acontinuing need to improve the clarity of color images produced bybinary color printers without decreasing speed.

Accordingly, it is an object of this invention to provide a binary colorprinter with a means for improving print image clarity.

It is yet another object of this invention to provide a binary colorprinter with means for automatically determining whether black orsecondary colors should be replaced with a combination of C,M or Y dots.

It is yet another object of this invention to provide a binary printerwith an improved noise dither process that is adaptive in accordancewith a characteristic of a color to be printed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high level block diagram showing a preferred embodiment ofthe present invention.

FIG. 2 is a high level flow diagram illustrating the color vector andhalftoning operations of the present invention.

FIG. 3 is a flowchart showing the conversion of the RGB color planes toHPG color vector.

FIG. 4 is a flowchart showing the color vector processing procedures ofthe present invention.

FIG. 5 is a schematic diagram illustrating the color vector processingprocedures of FIG. 3 when there is enough white for full black andsecondary color replacement.

FIG. 6 is a schematic diagram illustrating the color vector processingprocedures of FIG. 3 when there is enough white for full black, but onlypartial secondary color replacement.

FIG. 7 is a schematic diagram illustrating the color vector processingprocedures of FIG. 3 when there is enough white for for only partialblack and no secondary color replacement.

FIG. 8 is a schematic diagram showing the color vector ditheringprocedures of the present invention.

FIG. 9 is a flowchart showing an example of the color vector ditheringprocedures of the present invention.

SUMMARY OF THE INVENTION

A system selectively enables a printer to deposit on a sheet C,M, Y andK color dots at each of the plurality of pixel locations to produce acolor image. The system comprises a memory for storing C,M, Y and Kcolor values for each pixel in the color image and a processor forcontrolling selective deposition of the C,M, Y and K colors. Theinvention provides a system for selectively enabling a printer todeposit on a sheet, Cyan (C). Magenta (M), Yellow (Y) and Black (K)color dots at each of a plurality of pixel locations, to produce a colorimage, said system comprising: memory means for storing a first array ofcolor vectors for each pixel in the color image; processor means coupledto said memory means for determining a second array of color vectorsfrom aid first array of color vectors by (i) replacing the black colorcomponent of said first array with the primary colors cyan, magenta andblue for each pixel location from said color values stored in said firstarray, and (ii) replacing the secondary color component of said firstarray with the primary colors cyan, magenta and blue for each pixellocation from said color values stored in said first array.

The present invention provides a halftoning technique that producesprint quality near error diffusion with the speed of a dither bydithering on a color vector. Because of the replacement of black andsecondary colors the present invention produces better print qualitythan any other dither technique including noise dithering and often theprint quality is comparable to error diffusion. Moreover, the presentinvention has the speed advantage of a dither over error diffusion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a host processor 10 includes a central processingunit (CPU 12 which communicates with a random access memory (RAM) 14 viabus 16. A display 17 enables visualization of a color image from CPU 12.An input/output (I/O) module 18 enables data flow to a connected printer20. Printer 20 includes a CPU 22 and a print engine 24, both of whichact to provide binary dot pattern color images.

In RAM 14, a plurality of procedures and storage areas are includedwhich enable the system of FIG. 1 to carry out the invention hereof. RAM14 includes a CMM or color-matching module conversion procedure 26, andstorage areas 28, 30, 32 and 34 which respectively store R, G, B and Kpixel planes for a color image. RAM 14 further includes an RGB to colorvector procedure 36 that enables the content of each pixel in the colorimage stored in image planes 28, 30, 32 and 34 to be represented as acolor vector. A color vector processing procedure 38, and a halftoningprocedure 40 are also stored in RAM 14. Also stored in RAM 14 are HPGcolor vectors 42, six-component color vectors 44, and dither cell 46.

Initially, CPU 12, in conjunction with CMM conversion procedure 26,converts each red (R), green (G), blue (B) pixel value from an image ondisplay 17 to printer 24 RGB. During this conversion, an adjustment ismade to take into account any color variations from standard colors thatappear on display 17. A standard, 24-bit RGB triplet is passed in andconverted into a different RGB triplet by the color matching module 26conversion procedure. This color mapping procedure can be performedeither automatically by the CMM module or manually with userintervention. These procedures are described in U.S. patent applicationsADAPTIVE COLOR RENDERING BY AN INKJET PRINTER BASED ON OBJECT TYPE byThomas G. Smith, et al., Ser. No. 08/189,006, filed Jan. 27, 1994; COLORHALFTONING OPTIONS INFLUENCED BY PRINT MODE SETTING, by Kirt A. Winter,et al., Ser. No. 08/187,933, filed Jan. 27, 1994; MANUAL/AUTOMATIC USEROPTION FOR COLOR PRINTING OF DIFFERENT TYPES OF OBJECTS, by Sachin S.Naik, et al., Ser. No. 08/187,942, filed Jan. 27, 1994; and AUTOMATEDOPTIMIZATION OF HARDCOPY OUTPUT by Steven O. Miller, et al., Ser. No.08/188,618, filed Jan. 27, 1994; which are herein incorporated byreference.

Once the CMM conversion procedure has been completed, the image isrepresented by separate R, G, and B image planes with each image planeincluding an 8-bit pixel value for each pixel location evidencing theintensity of the respective color for that pixel location. Thus, redplane 28 will have an 8 bit value indicative of the red intensity, greenplane 30 will have an 8-bit value indicative of the green value, etc. Anadditional K (black) plane, 34 includes an 8-bit K value for each pixellocation. Before the respective RGB and K planes are passed to CPU 22 inprinter 20, they are subjected to at least three procedures which enablean adjustment of the values of the respective colors to improvepresentation in the image. Those procedures are RGB to color-vectorprocedures 36, color-vector processing procedures 38 and halftoningprocedures 40. In FIG. 2, the above procedures are illustrated by a flowdiagram.

Before describing the color-vector conversion (CVC) procedure 36, thecolor-vector processing (CVP) procedure 38 and the color-vectordithering procedure 40, a description of the Hue-Plus-Gray (HPG) colormodel and color-vector component thereof will be discussed, since thisinvention is best described in terms of the HPG color model. HPGincorporates the concept that renderings of a specific hue may becompletely achieved by combining a quantity very closely related to thechroma of that hue with various levels of gray.

Color itself can be described according to three differentcharacteristics. These do not take into account all the variables ofcolor, but do handle the subject sufficiently to explain color printing.The "hue" of color is the actual color appearance, i.e. red, green,purple, orange, blue-green, etc. The hue is the characteristic whichgives color a basic name. The second characteristic comes from the factthat some colors cannot be classified as hues, i.e. black, gray, andwhite. These are called achromatic colors. The presence of gray in acolor is a measurement of the "chroma" and can be described as thecolor's intensity or saturation. The more gray, the less intense andvice versa. The third characteristic is defined as "value" in theMunsell color system and describes the color's lightness or darkness.Thus, you can have a light blue or a dark green, and both can be intense(lacking gray) in reference to their chroma.

Color space is a system for mathematically defining color. Manydifferent color spaces exist, including RGB (red, green, blue), CMYK(cyan, magenta, yellow, black), and numerous device-independent colorspaces such as Munsell, CIE XYZ and CIE L*a*b.

HPG is simultaneously a color space, a machine-space color vector, and acolor-control concept that increases color control. It is a polarcoordinate space and is designed to be consistent with the triangularshape of color palettes. In this model, color is divided into its twomajor components, chromatic and achromatic. Each of these components inturn is subdivided into two respective subcomponents. The chromaticcomponent is divided into two fractional components consisting of twocolorants called either primary and secondary, or dominant andsubordinate primaries.

The chromatic component controls hue and chroma. Hue is controlled bycombining only two colorants. The hue coordinates of the two colorantsdetermine the range of hues that can be reproduced by those twocolorants. Specifically, the range is restricted to only those huespositioned between the hues of the two colorants. As an example, huesthat range from colorant 1 (here abbreviated "C1") through colorant 2("C2") may be achieved by applying quantities of C1 and C2 in which thefractional amounts of each range between zero and one, in inverserelationship, but summing to one.

Two favorable results are achieved by restricting the hues to thosewhich are between the hues of the two colorants employed: (1) thepossibility of reproducing a hue by using colorants which are furtheraway from the desired hue is removed. At the rendition stage, thisremoves the possibility of hue artifacts. (2) the ability to controlhues is increased, reducing hue errors.

Chroma is directly proportional to one coordinate of the HPG system,namely the quantity of colorant present. Chroma is controlled bycontrolling the quantity of colorant applied to the medium. Chroma isdifficult to control when based only on the chroma of the primaries andsecondaries (or colorants). For pixel-based color printing machines thecolorant in each pixel overlaps slightly with the colorant from adjacentpixels. The resulting chroma is determined in part by the effectivechroma for the hue that arises from mixing of the two colorants wherethey overlap. This effective chroma differs from the superposition oraverage of the two or several chromas nominally established by theapparatus for the primaries and secondaries in use. The chroma componentof HPG can be correlated or controlled as a function of hue.

The achromatic component of color is described by a single variable,value V or lightness. Achromatic colors, called grays, are measured byusing intermediate numbers along those scales between the high and lowextremes white W and black K. True grays have zero chroma and hue.

In the HPG system the control of value or lightness is achieved bycontrolling the quantity of gray to be applied. In turn, a specific grayis achieved by controlling the quantity of black used. Thus, thequantity of black is also a coordinate of the system.

The complete HPG model now will be described in terms of its individualcomponents. The color space is described fractionally so that it can beeasily scaled to the color primaries of any specific delivery system.The color space is divided into two components: the fractional partF_(c) which is chromatic and the fractional part F_(a) which isachromatic. In this document the chromatic fraction is also called"Fraction-Colorant" and denoted by the symbol N. The sum of thechromatic and achromatic components produces unity:

    F.sub.c +F.sub.a =N+F.sub.a =1.

The chromatic portion F_(c) =N is the principal parameter for control ofchroma. It is further subdivided into two components C1, C2 to controlhue. The fractional parts F_(c1) and F_(c2) required for hue control arerequired to fill the chromatic space:

    F.sub.c1 +F.sub.c2 =F.sub.c =N.

The achromatic or gray component F_(a) is further divided into twocomponents to control value. The fractional parts of black K and white Ware required to fill the achromatic space:

    F.sub.k +F.sub.w =F.sub.a

In this document the variable F_(k) is called "Fraction-Black" and isalso denoted by the symbol K.

The complete model of any color can be expressed as:

    F.sub.C1 +F.sub.c2 +F.sub.k +F.sub.w =1

This form is called an HPG color vector and is in a form suitable forhalftoning either by ordered dither or error diffusion.

The HPG system permits direct control of the three major colorattributes: value or lightness, hue, and chroma or vividness. Value orlightness is controlled by applying gray fractionally, e.g., bydelivering black on a white medium. Hue is controlled as relativefractional proportions of two colorants and chroma is controlled byapplying this hue fractionally.

More specifically, hues are created using fractional combinations ofadjacent dominant and subordinate primary colorants C1, C2. These areselected to most closely enclose the target hue. The resulting color isshaded by inserting gray in place of some of the chromatic colorants,black to darken the color, white to lighten it. The total amount N ofchromatic colorant is directly proportional to the conceptual chromaparameter V in idealized Munsell HVC space.

Because of good correlation or consistency with color-reproductionprocesses, the HPG system variables H, N, K or their elements F_(c1),F_(c2), F_(k) can be applied with little variation in the processdevices. Because the color is coded in terms which apply directly to thedelivery mechanism, the HPG parameters can be halftoned directly.

Color Vector Conversion

The RGB to color vector conversion procedure 36 in RAM 14 converts theRGB triplet into an HPG color vector. A color vector is the sum of nvector components each with a color (direction) and magnitude. Asdiscussed above the HPG color vector is a 4 component color vector. Eachcomponent has a magnitude in the range 0-255 with the additionalconstraint that for a given pixel all the components of the color vectoradd up to 255. FIG. 3 is a flowchart showing the conversion of the RGBcolor planes to HPG color vector. In step 100, the white component ofthe color vector, w, is set a value equal to the value of the RGBcomponent having the smallest value, i.e., w=min{R,G,B}. In step 110,the primary component (cyan, magenta or yellow) of the color vector, p,is set to a value equal to the difference in value between RGB componenthaving the middle value and the RGB component having the minimum value,i.e., p=mid{R,G,B}-min{R,G,B}. In step 120, the secondary component(red, green or blue) of the color vector, s, is set to a value equal tothe difference in value between the RGB component having the largestvalue and the RGB component having the middle value, i.e.,s=max{R,G,B}-mid{R,G,B}. Finally, in step 130, the black component ofthe color vector, k, is set to a value equal to the difference in valuebetween 255 and RGB component having the largest value, i.e.,k=255-max{R,G,B}. The HPG color vector is a 4-component color vectorthat will have a black component, a white component, a primary colorcomponent (cyan, magenta or blue), and a secondary color component (red,green, or blue), any of which may have a zero value.

The HPG color model and color vectors are described in greater detail incopending commonly assigned U.S. patent application APPARATUS FORFORMING COLOR IMAGES USING A HUE-PLUS-GRAY COLOR MODEL by PAUL H.DILLINGER, Ser. No. 07/878,931, filed May 5, 1992, and now issued asU.S. Pat. No. 5,377,024 which is herein incorporated by reference.

Color Vector Processing

The color vector is then processed by the color vector processingprocedures 38 stored in RAM 14 into a different color vector. Typically,eight colors can be printed (R, G, B, C, M, Y, K, W). The CV Processing38 translates the four-component HPG color vector into a 6-componentcolor vector from the 8 possible color components R, G, B, C, M, Y, Kand W, with the restriction that the color vector will have a maximum ofone secondary color component, (i.e., either a red, green, or bluesecondary color component), or it may have no secondary component (i.e.,a zero value for the secondary color component). The objective of thecolor-vector processing is to minimize the contrast between dots on thepage and adjacent areas which improve the printed image by providing abetter simulation of continuous tone. A replacement scheme for black andsecondary colors that accomplishes that objective is embodied in thepresent invention, wherein medium to light colors have primary (C,M,Y)dots and dark colors preserve secondary (R,G,B) and black dots.

In converting the four-component HPG color vector into a 6-componentcolor vector, the color-vector processing procedures do the following:(1) black is replaced with certain levels of cyan, magenta, yellow, andblack, and (2) secondary colors (i.e., red, green, blue) are replaced bycertain levels of themselves (i.e., red, green, blue) and their primarycomponents (i.e., cyan, magenta, yellow). In performing the abovereplacements, all or part of the white component of the color vector isreplaced.

For black, the color vector processing procedures attempt to replace ablack dot with separate cyan, magenta, and yellow dots, Thus, if theblack value in the color vector is "k" then the black component is madezero, i.e., make k=0, and add that value, δp=k, to the cyan, magenta,and yellow primary components. However, there is the restriction thatthe sum of the components of the color vector must total 255. To satisfythis restriction, the color-vector processing procedures subtract 2*δpfrom the white component. Therefore, in order for the procedure to work,the white component must must be greater than or equal to twice theblack component, i.e., so long as w≧2*k. The way to ensure maximumreplacement in all situations, is to make δk=min{k,w/2} instead of thevalue k. Referring to FIG. 4, in step 200 the black component k isreduced by δp=δk=min{k,w/2}, i.e., k=k-δk=k-δp. In step 210, the primarycomponents, p, are increased by the amount δp=δk, i.e., c=c+δc, m=m+δm,and y=y+δy; where δc=δm=δy=δp. Thus, for dark colors, the whitecomponent, w, will be small and little replacement will occur. However,for light colors w will be large and full replacement of k will beallowed. These replacement procedures also cause a smooth transition inthe middle colors.

After performing the replacement for the black component of the colorvector, replacement for the secondary (R,G or B) component of the colorvector is performed. Since the initial color vector is an HNK4-component color vector it will have a maximum of one secondary colorcomponent, (i.e., either a red, green, or blue secondary colorcomponent), or it may have no secondary component (i.e., a zero valuefor the secondary color component). In the following discussion, thevalues for w, p₁, P₂ and s are the resulting values after blackreplacement. In step 220, the secondary color component, s, which can beeither "r", "g", or "b", is reduced by δs, i.e., s=s-δs; whereδs=min{s,w}. In step 230, the primaries p₁, P₂ used to make up thatsecondary, s, which can be either "c", "m" or "y" are then increased byδs, i.e., P₁ =P₁ +δp₁ ; P₂ =P₂ +δP₂ ; where P₁ and P₂ are "m" and "y"for red, "c" and "y" for green, and "c" and "m" for blue, respectively,and δp₁ =δp₂ =δs. White is then reduced by δs, i.e., w=w-δs, to preservethe restriction that the sum of the components of the color vector musttotal 255. The secondary replacement is complete and the color vectorprocessing is completed according to the replacement rules of thepresent invention.

The black component of the color vector is replaced first because theblack dots are darker and therefore more offensive with respect toattempting to represent continuous tone in the lighter shades. Any valueof the white component of the color vector that remains after blackreplacement is completed, is then available for secondary colorreplacement.

In an alternative embodiment of the present invention, the replacementrules are modified so that less replacement is performed when colorblotching is likely. With a slight modification to the black replacementimplementation, the replacement in the majority of mid-tones isrestricted while preserving full replacement at low levels where it ismost important. This is accomplished by instead of using the replacementvalue δk=min{k, w/2}, δk=min{k, w/α} is used. A similar change is madeto the secondary color replacement. Instead of δs=min{s, w}, δs=min{s,w/β}. Values of α=16 and β=8 have been found to work well.

These two simple changes cause a dramatic effect on the output. Forlarge values of w, when using α=16 β=8, δk and δs still exceed k and s,respectively, and full replacement is performed, but as the midtones areapproached, using this alternative limitation causes more black andsecondary dots to remain and any color blotching is minimized.

FIG. 5 is a schematic diagram illustrating the color vector processingprocedures when there is enough white for full black and secondary colorreplacement. FIG. 6 is a schematic diagram illustrating the color vectorprocessing procedures when there is enough white for full black, butonly partial secondary color replacement. FIG. 7 is a schematic diagramillustrating the color vector processing procedures when there is enoughwhite for only partial black and no secondary color replacement. Thedetails of these examples are evident from the Figures and noexplanation is required.

Color Vector Dithering

Color printers typically can print one of eight colors at a particularpixel (red, green, blue, cyan, magenta, yellow, black, or white).However, the computer can request any one of 16 million colors. (Shadesof 0-255 for Red, Green, and Blue produce 16 million combinations.)Therefore, it is necessary to produce a translation between 24-bitpixels (16 million colors) and 3-bit pixels (eight colors). As discussedabove, this translation is called halftoning. An embodiment of theinvention includes a method of halftoning, which comprises dithering onthe 6-component color vector described previously. The following is adescription of halftoning the HPG color vector using a noise cell. Thedither cell is then used to select one of those six components. Thecomponent thus selected is the single color to be printed at thatparticular printer pixel. Dither cells are customarily placed initiallyin a position where the upper left corner of the cell corresponds to theupper left corner of the image being rendered. This placement causeseach dither cell location to now correspond to specific pixels in theimage area covered by the dither cell. The color printed by thehalftoning process is determined by the comparison of the color vectorof the pixel, and the value of the dither cell that corresponds to thatpixel. Rendering of that area of the image corresponding to the areacovered by the dither cells is performed. The dither cell is nowre-positioned over the image by shifting the cell to the right by theamount which corresponds to the width of the cell. This new placement ofthe cell covers a new image area immediately adjacent to the area of theimage previously covered. This part of the image may then be rendered.The dither cell is moved again and again until the right hand side ofthe image is reached. The process continues by returning the dither cellto the left side of the image and shifting it downward by the amountwhich corresponds to the height of the cell. The process is thenrepeated until the entire area of the image has been covered andrendered. Referring to FIG. 8, shown is the procedure for comparing thedither cell to the color vector. The decision steps, for each pixel areshown in the flowchart of FIG. 9 and need no further explanation.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. While the invention has been described with specificity, itwill be understood by those skilled in the art that substitution ofequivalents and other variations there-of may be made without departingfrom the spirit of the invention or the scope of the appended claims. Inaddition, while the invention has been described with respect to a colorink jet printer, it is equally applicable to other color outputapparatus which is required to perform a color transformation.Accordingly, the present invention is intended to embrace all suchalternatives, modifications and variances which fall within the scope ofthe appended claims.

What is claimed is:
 1. A method for printing a binary-dot simulation ofa continuous-tone color image, said method comprising the stepsof:receiving or generating a multilevel color representation of thecontinuous-tone color image at each of a multiplicity of binary-dotpositions; replacing a quantity of a relatively dark color in themultilevel color representation with an equal quantity of each of pluralrelatively light colors, to form a new multilevel color representation;deriving a binary color representation from the new multilevel colorrepresentation; and applying the binary color representation to controla binary printer.
 2. The method of claim 1, wherein:the relatively darkcolor is black.
 3. The method of claim 1, wherein:the replacing stepincludes one iteration, replacing a quantity of a relatively light colorwith plural quantities of even lighter colors to form the new multilevelcolor representation.
 4. The method of claim 3, wherein:the relativelylight color replaced in the iteration is a subtractive secondary.
 5. Themethod of claim 3, further comprising:removing a corresponding quantityof white; wherein the corresponding quantity is either equal to or amultiple of the replaced quantity of relatively dark color, dependingupon the number of said plural relatively light colors.
 6. The method ofclaim 3, wherein:the replacing step comprises controlling the replacedquantity of relatively dark color in dependence upon a quantity of whiteavailable for removal.
 7. The method of claim 6, wherein:the controllingstep comprises setting the replaced quantity equal to the smaller of (a)the quantity of the relatively dark color available for replacement and(b) a function of the quantity of white available for removal.
 8. Themethod of claim 7, wherein:in the iteration, said function does notexceed the quantity of white available; and in the first performance ofthe replacing step, said function is a fraction not exceeding one-halfof the quantity of white available.
 9. The method of claim 7, wherein:inthe iteration, said function is between one-eighth and one times thequantity of white available; and in the first performance of thereplacing step, said function is a fraction between one-sixteenth andone-half of the quantity of white available.
 10. The method of claim 3,wherein:the even lighter colors are subtractive primaries.
 11. Themethod of claim 1, further comprising:removing a quantity of white thatis either equal to or a multiple of the replaced quantity of relativelydark color, depending upon the number of said plural relatively lightcolors.
 12. The method of claim 11, wherein:the replacing step comprisescontrolling the replaced quantity of relatively dark color in dependenceupon a quantity of white available for removal.
 13. The method of claim1, wherein the receiving-or-generating step comprises:receiving orgenerating a first multilevel color representation in terms of additivecolor primaries; and developing therefrom a second multilevel colorrepresentation in terms of a color vector.
 14. The method of claim 1,wherein:the developing step comprises forming a color vector in thehue-plus-gray color system.
 15. Apparatus for printing a binary-dotsimulation of a continuous-tone color image, said apparatuscomprising:means for receiving or generating a multilevel colorrepresentation of the continuous-tone color image at each of amultiplicity of binary-dot positions; first programmed digital processormeans for reducing contrast in relatively light regions of the image byreplacing a quantity of a relatively dark color, exclusively forbinary-dot positions that are in relatively light regions of themultilevel color representation, with an equal quantity of each ofplural relatively light colors, to form a new multilevel colorrepresentation; second programmed digital processor means for deriving abinary color representation from the new multilevel colorrepresentation; and nonvolatile memory means holding instructions foroperation of the processor means.
 16. The apparatus of claim 15,wherein:the first programmed digital processor means comprise means forselecting black for replacement as the relatively dark color.
 17. Theapparatus of claim 15, wherein:the first programmed digital processormeans comprise means for including one iteration in said replacing,namely replacing a quantity of a relatively light color with pluralquantities of even lighter colors to form the new multilevel colorrepresentation.
 18. The apparatus of claim 17, wherein:theiteration-including means comprise means for selecting a subtractivesecondary for replacement as the relatively light color replaced in theiteration.
 19. The apparatus of claim 15, further comprising:means forremoving a quantity of white that is either equal to or a multiple ofthe replaced quantity of relatively dark color, depending upon thenumber of said plural relatively light colors.
 20. The apparatus ofclaim 19, wherein:the removing means comprise means for controlling thereplaced quantity of relatively dark color in dependence upon a quantityof white available for removal.